If your mother's place is a borderline case (ie many people there or is very small) then it is probably a good idea to obtain one from your local council. Here's a link for the service in Scarborough (you will need to google for your local council)
If though for example there's just the two of you living in a 3 bedroom house I wouldn't bother.
It is a two bedroom place, with two double rooms. my mum and her partner in one and my self in the other. we intend to stay here up until the marriage and then find a rented house.